This invention is related to bearing means for high temperature applications, and more particularly to a bearing useful for supporting a steel mill roll transferring a hot steel strip.
Rolls are used in steel mills for transferring hot strips of steel in an environment where the strip may be 1800.degree. F. to 2200.degree. or more. The roll and the supporting bearings become very hot. The temperature varies substantially as the furnace is opened and closed.
Roller bearings are commonly used for such applications. Each bearing supports a short shaft that extends from each end of the roll.
Roller bearings have a relatively limited life for several reasons. One reason is that the rollers in the bearing are mounted between an inner race and an outer race. Since the diameter of the inner race is less than the diameter of the outer race, the rollers must slide in one direction on the inner race or in the opposite direction on the outer race. The sliding develops facets in the races, requiring the rollers to jump over each facet, and causing the rolls to vibrate. This is sometimes referred to as "skidding wear".
When the rollers are started, they are dry and hot, that is 400.degree.-600.degree. F. Rollers formed of a hard steel tend to corrode because of the high operating temperature. Other materials, such as stainless steel are too soft and tend to gall.
The steel strip load tends to flex the roll. The flexing roll causes each end shaft to shift slightly in its bearing. Roller bearings have a low tolerance for such shifting. Flexing also occurs when the roll returns to its original shape as it is unloaded. This is a vertical flexing. In some cases, a horizontal flexing occurs because some of the rolls may be rotating slower than others. A steel strip pushing a slower roll forward, causes a forward horizontal flexing. A reverse horizontal flexing takes place when the roll velocity is higher than the strip velocity.
Still another problem is caused by inadequate bearing lubrication. High temperature lubricants are commonly used, however, they tend to cake-up. Maintenance personnel try to pump out the caked-up lubricant and replace it with new lubricant.
Usually when the roll stalls due to a bearing failure, the roll is distorted. Frequently an expensive roll is lost because of a relatively, short-lived, less expensive bearing.